This proposal will test the hypothesis that glutamatergic signaling orchestrates early stages of granule cell development necessary for proper cerebellar circuit formation. Glutamate, which has a conserved signaling function across phyla, controls multiple intracellular signaling pathways through diverse receptors. Glutamate receptor expression in the brain is tightly developmentally regulated, suggesting specific functions of each receptor subtype at different stages of cell development. Indeed, emerging evidence implicates glutamate receptors in brain development. However, our understanding of such functions lags behind the known diversity of glutamate receptor subtypes. Previous and preliminary data suggest that granule cell precursors (GCPs) transiently express functional GluK2-containing kainate receptors (KAR) and group I metabotropic glutamate receptor 5 (mGlu5R) in the external germinal layer (EGL) when they exit the cell cycle and extend axons. However, it is not known whether, and if so how, gain or loss of function in GluK2 receptor and mGlu5R alters GCP development. In this proposal, Aim 1 and Aim 2 will examine the function of GluK2 and mGlu5R on GCP proliferation and axon extension, respectively. Aim 3 will explore whether Bergmann glial cells, a specialized astrocytes extending processes through the EGL, release glutamate controlling GCP cell-cycle exit through mGlu5R. We will use a combination of time- lapse confocal microscopy (for calcium imaging, migration, and axon extension) in acute slices and in vivo approaches (drugs, RNA interference via electroporation, and genetic manipulation) in wild-type and transgenic mice, including mGlu5Rfl/fl (fl, floxed) and GluK2- knockout (KO).